/ ˌʌndəˈlaɪɪŋ; NAmE -dərˈl-/ adjective [ only before noun ]
1.
important in a situation but not always easily noticed or stated clearly :
The underlying assumption is that the amount of money available is limited.
Unemployment may be an underlying cause of the rising crime rate.
2.
existing under the surface of sth else :
the underlying rock formation
—see also underlie